Individual nanostructures, as well as those embedded in other materials to form nanocomposite materials, have many promising applications, including applications that make use of their optical and electronic properties. One particularly useful application would be in the area of nanocomposite based memory, where the nanostructures allow for high density charge storage.
Of the synthetic approaches available for preparing nanostructures, top-down patterned approaches such as chemical vapor deposition (CVD) or molecular beam epitaxy (MBE) are commonly used to generate core and core:shell nanostructures. These methods typically yield large and/or disordered and/or low density packing nanoparticles, and require high cost (high temperature, high vacuum) processing steps. Solution based syntheses can also be used to synthesize semiconductor nanocrystals (either cores or core/shells) which are more readily compatible with solution based deposition methods such as spin coating or other evaporation methods. For example, nanostructures comprising CdSe cores (or crystalline cores) with a shell of ZnS can be prepared by solution deposition techniques (see, for example, Murray et al (1993) “Synthesis and characterization of nearly monodisperse CdE (E=S, Se, Te) semiconductor nanocrystals” J. Am. Chem. Soc. 115: 8706-8715). However, nanostructures generated by these and other standard core-shell synthetic techniques typically do not have a thick enough shell to confine a charge in the core to enough degree to prevent charge diffusion to other nanostructures placed within a few nanometers of the first nanostructure.
Alternatively, nanostructure synthesis by a chemical self-organizing approach typically produces the most well-controlled morphology and crystal size, but these synthetic protocols generate nanostructures having associated therewith additional organic and/or surfactant compounds. While useful for enhancing solubility and facilitating manipulation of the nanostructures during synthesis, the organic contaminants are avidly associated with the nanostructure surface, thus inhibiting further manipulation and/or integration of the newly synthesized nanostructure into devices and end applications.
Even if these CdSe:ZnS constructs could be prepared having diameters allowing for high density packing (e.g., about 1×1012/cm2 or greater), the ZnS shell would not provide enough quantum confinement for efficient use of the nanostructures in microelectronic and photonic devices, including, but not limited to, memory or charge storage devices.
Accordingly, there exists a need in the art for discrete coated nanostructures that can be easily integrated into various manufacturing processes without further processing. Preferably, the coated nanostructures can be closely packed while maintaining greater quantum confinement than standard CdSe/ZnS core:shell structures.
In addition, the energy levels (electron affinity) of component semiconductor materials are an important consideration for fabrication of semiconductor-containing devices, such as photovoltaic devices, memory storage devices, transistors, and light-emitting and/or light-detecting devices, such as LEDs, phosphors, photo-detectors, and the like. Bulk semi-conductors have inherent valence and conduction bands associated with the specifics of the atomic composition. However, nanocrystals constructed of the same material(s) are thought to differ in energy levels compared to their bulk counterparts, due at least in part to the effects of quantum confinement; the energy levels can also be tuned for a given material, e.g., by variation of the nanocrystal size. Matching appropriate materials and energy level alignment is considered important for optimal device performance. Accordingly, there exists a need in the art for techniques that can be used to match appropriate materials and energy levels.
The present invention meets these and other needs by providing discrete coated nanostructures, ligands for coating discrete nanostructures, devices incorporating the coated nanostructures, and methods for preparing the coated nanostructures. A complete understanding of the invention will be obtained upon review of the following.